Why Does Lewis Start With the Law of Human Nature?

Lewis is in effect using a gateway argument. In effect, an argument or reflection which in effect draws people in. And what Lewis is saying is once you follow this line of thought through, you are going to end up in a very Christian place. It's all about the opening pages of my Christianity is how on earth we settle arguments. Then Lewis begins to make the connections which move his argument on. Why does he think that we all have this inherent moral law inside us? How does he articulate that? Well Lewis has this deep sense that this is the way things are. Not everyone does. Some would say, well, look, I don't feel like that but most people usually do